When an energy-storable phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storable phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storable phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is then subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storable phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of phosphor layers are known and used. For example, a phosphor layer comprising a binder and an energy-storable phosphor dispersed therein is generally used, and a phosphor layer comprising agglomerate of an energy-storable phosphor without binder is also known. The latter layer can be formed by a gas phase-accumulation method or by a firing method. Further, still also known is a phosphor layer comprising energy-storable phosphor agglomerate impregnated with a polymer material.
Japanese Patent Provisional Publication 2001-255610 discloses a variation of the radiation image recording and reproducing method. While an energy-storable phosphor of the storage panel used in the ordinary method plays both roles of radiation-absorbing function and energy-storable function, those two functions are separated in the disclosed method. In the method, a radiation image storage panel comprising an energy-storable phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of causing the radiation-absorbing phosphor of the screen (and of the panel) to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy-storable phosphor of the panel to store the energy of the converted light as radiation image information; sequentially exciting the energy-storable phosphor with a stimulating ray to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric signals giving a visible radiation image.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. It is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
In order to improve the sensitivity and the image quality, it is proposed that the phosphor layer of the radiation image storage panel be prepared by a gas phase-accumulation method such as vacuum vapor deposition, sputtering or chemical vapor deposition (CVD). The process of vacuum vapor deposition, for example, comprises the steps of: heating to vaporize an evaporation source comprising a phosphor or starting materials thereof by means of a resistance heater or an electron beam, and depositing and accumulating the vapor on a substrate such as a metal sheet to form a layer of the phosphor in the form of columnar crystals.
The phosphor layer formed by a gas phase-accumulation method contains no binder and consists of phosphor only, and there are gaps among the columnar crystals of the phosphor. Because of the presence of gaps, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently. Accordingly, a radiation image storage panel having that phosphor layer has high sensitivity. At the same time, since the gaps prevent the stimulating light from diffusing parallel to the layer, the radiation image storage panel can give a reproduced radiation image of high sharpness.
Japanese Patent Provisional Publication 2003-050298 discloses a process for preparation of a radiation image storage panel. In the disclosed process, first a layer of phosphor matrix alone in the form of columnar crystals is formed by a gas phase-accumulation method, and then another layer made of the phosphor is formed thereon (so that each columnar crystal of the phosphor may be one-to-one grown on each columnar crystal of the phosphor matrix), to prepare a phosphor layer excellent in columnar crystallinity.
Japanese Patent No. 2,514,322 discloses a radiation image storage panel improved in both of sharpness and sensitivity. The disclosed storage panel comprises a stimulable phosphor layer and a light-scattering layer provided thereon. In the phosphor layer, columnar crystals of stimulable phosphor are aligned. The light-scattering layer has a rough surface, and is placed on the side opposite to the side where stimulating light comes. Disclosed materials for the light-scattering layer are pigments, porous metals, sand-ground metals, ceramics, frosted glass and opal glass.